Delayed coking involves thermal decomposition of petroleum residua (resids) to produce gas, liquid streams of various boiling ranges, and coke. Delayed coking of resids from heavy and heavy sour (high sulfur) crude oils is carried out primarily as a means of disposing of these low value resids by converting part of the resids to more valuable liquid and gaseous products, and leaving a solid coke product residue. Although the resulting coke product is generally thought of as a low value by-product, it may have some value, depending on its grade, as a fuel (fuel grade coke), electrodes for aluminum manufacture (anode grade coke), etc.
In the delayed coking process, the feedstock is rapidly heated in a fired heater or tubular furnace. The heated feedstock is then passed to a large steel vessel, commonly known as a coking drum that is maintained at conditions under which coking occurs, generally at temperatures above about 400° C. under super-atmospheric pressures. The heated residuum feed in the coker drum generates volatile components that are removed overhead and passed to a fractionator, ultimately leaving coke behind. When the coker drum is full of coke, the heated feed is switched to a “sister” drum and hydrocarbon vapors are purged from the drum with steam. The drum is then quenched by first flowing steam and then by filling it with water to lower the temperature to less than about 100° C. after which the water is drained. The draining is usually done back through the inlet line. When the cooling and draining steps are complete, the drum is opened and the coke is removed after drilling and/or cutting using high velocity water jets.
For example, a hole is typically bored from the top of the drum through the center of the coke bed using water jet nozzles located on a boring tool. Nozzles oriented horizontally on the head of a cutting tool then cut the coke from the drum. The coke removal step adds considerably to the throughput time of the overall process. Thus, it would be desirable to be able to produce a free-flowing coke, in a coker drum, that would not require the expense and time associated with conventional coke removal, particularly the need to drill-out the coke. It would also be desirable to be able to safely remove such substantially free-flowing coke at a controlled flow rate.
One problem associated with removing free-flowing coke from a coke-drum is controlling its removal from the drum. Coke drums are typically large cylindrical vessels, commonly 19 to 30 feet in diameter and two to three times as tall having a top head and a funnel shaped bottom portion fitted with a bottom head. They are usually used in pairs so that they can be operated alternately. That is, one drum can be on-line while coke is being removed from the other. The heads of a conventional coke drum must be removed to remove the coke. The process of removing and replacing the removable top head and bottom head of the vessel cover is called heading and unheading or deheading. It is dangerous work, with several risks associated with the procedures. There have been fatalities and serious injuries during such procedures. Operators face a significant safety risk from exposure to steam, hot water, fires and repetitive stress associated with the manual unbolting work. Accordingly, the industry has devoted substantial time and investment in developing semi-automatic or fully automatic unheading systems, with attention focused on bottom unheading where the greatest safety hazard is present.
Additionally, if loose coke is let out from the bottom of a coke drum in a rapid and uncontrolled fashion, significant problems can occur. For example, if the flow is too rapid, and the drum top head and/or vent lines are not open, a vacuum can be pulled on the coke drum, imploding the coke drum. Also, rapid dumps of large drums of coke, e.g, dumping 1000 tons (1016.05 Mg) of coke plus its interstitial water in less than about 5 or 10 minutes, can cause significant structural damage to chutes and coke receiving areas.
There are several conventional methods for removing the bottom head of a coker drum out of the way of the falling coke. One method is to completely remove the head from the vessel, perhaps carrying it away from the vessel on a cart. Another method is to swing it out of the way, as on a hinge or pivot, while the head is still coupled to the vessel as in U.S. Pat. No. 6,264,829, which is incorporated herein by reference. Conventional systems all use a manual or semi-automatic bolting system that must be uncoupled with every decoking cycle.
Also, conventional bottom head removal systems require that the heated feed enter the coke vessel from the bottom through the center of the bottom head. Thus, in the typical commercial delayed coker operation, before removing the vessel bottom head for decoking, the feed line must first be disconnected before the bottom head can be removed. Finally, in many coker operations, a coke chute must be manually or hydraulically moved into place and, typically, safety bolts are manually inserted to secure the chute to the drum, allowing the chute to receive the falling coke. The chute directs the coke, as it is drilled out of the vessel, to a receiving area where it is later removed. These methods still require the feed line to be opened up and the head removed before the bottom chute can be brought up and attached to the bottom flange of the vessel.
Considering that there is exposure to personnel and/or equipment when opening the feed line, and considering there is exposure to personnel and/or equipment when opening the bottom head before the chute comes up and is attached, and considering there may still be personnel exposure to steam/hot water between the chute and bottom head after the chute is up, improvements in the coke vessel bottom unheading system to allow safe removal of coke from the vessel is highly desirable, particularly when the coke is a substantially free-flowing coke.
U.S. Patent Application No. 2003/0127314 A1, which is also incorporated herein by reference, teaches a process and apparatus for removing coke from a delayed coker vessel without unheading the vessel bottom. This is accomplished by feeding the resid feedstock into the side of the bottom section of the coker drum and using an aperture closure unit fitted and sealed to the bottom of the coker drum, which aperture closure unit is used to empty the drum of coke. There is no discussion of coke morphology, no suggestion that the coker can contain any quantity of free-flowing coke or that it be in the form of an aqueous slurry, or that the aperture closure member can be throttled to allow the controlled discharge of free-flowing coke in a safe manner.
While there are various teachings in the art for removing coke from coker drums and for various drum hardware solutions, there still remains a need in the art for improved methods of more efficiently emptying free-flowing portions of coke from the coke drum.